Since its appearance in the United States in 1999, West Nile virus (WNV) has caused the deaths of large numbers of birds and horses and close to 1000 humans. There is currently no effective therapy available, and a potent therapy is urgently needed. The goal of this project is to develop and evaluate the efficacy of recombinant human monoclonal antibodies for the prophylaxis and treatment of lethal WNV infection. We have produced recombinant human monoclonal single chain variable fragments (scFv) against the WNV envelope protein using a phage display screen, and characterized these antibodies both in vitro and in vivo. This procedure does not require immune selection, and can identify novel epitopes that are not normally recognized during infection or immunization with antigen - something that is not possible with other antibody development strategies. We have shown that these antibodies - particularly scFv #11, our model antibody for development - neutralize WNV and related flaviviruses in vitro, and are both protective before infection and therapeutic after infection in the mouse model of WNV. Here we propose to produce scFv#11 as a complete antibody and demonstrate its efficacy against WNV, as well as its cross-protective neutralization of dengue virus and other flaviviruses. In addition, we will produce and characterize additional recombinant antibodies to develop a synergistic combination of two antibodies for enhanced neutralization and therapy against WNV and other flavivirus infections. The recombinant antibodies will be fully human, produced rapidly and in high titer, free of blood-borne pathogens, and will be promising candidates for further development as a clinical reagent to combat flaviviral infection.